This invention pertains to fabrication of expanded multisheet metallic structures, and more particularly to apparatus and methods for superplastic forming of a pack made up of multiple sheets of superplastic metal, welded together using laser welding and expanded in a preheated die using a precisely controlled gas forming schedule.
Multisheet superplastically formed, diffusion bonded, expanded metal sandwich structures have been in use for many years, primarily in the aerospace industry, because of the low cost, high temperature capability and good strength and stiffness per unit weight that these structures offer. Various processes for fabricating these structures have been developed in the past, with various degrees of success, but all have proven costly and slow to produce, and often they have been prone to produce defective or unreliable parts.
Most of the existing techniques for fabricating such structures, including the truss core technique shown in U.S. Pat. No. 3,927,817 (Hamilton), use superplastic forming of a stack of sheets in a die having a cavity shaped like the final sandwich structure. The stack includes one or more core sheets that are selectively joined to each other, when there is more than one core sheet, and to a top and bottom sheet that form the top and bottom outside skins of the sandwich structure. The stack is inflated at superplastic temperature with gas pressure to expand the top and bottom sheets outwardly against the interior walls of the die cavity with gas pressure in a die cavity to the desired exterior dimensions. During superplastic forming, the core sheets stretch between their attachment areas to the top and bottom skins as those skins expand toward the boundary surfaces of the die cavity.
Early developments of techniques for fabricating multi-sheet expanded metal sandwich structures utilized diffusion bonding to join the core sheets along selective areas to produce the desired core structure. These techniques require the accurate placement of stop-off to prevent diffusion bonding in areas where adjacent sheets were not intended to bond together. Diffusion bonding is a desirable joining method because the junction retains superplastic qualities, but it has been difficult to produce a clean junction line free of stop-off that is narrow enough, and diffusion bonding can be a lengthy process with long holding times in the press at elevated temperatures, preventing the press from being used for other production. The capital intensive and time consuming nature of the diffusion bonding process lead to research into other techniques for joining the core sheets of multisheet stack that would be faster, more reliable, and less costly.
Another joining method, shown in U.S. Pat. Nos. 4,217,397 and 4,304,821 (Hayase et al.), uses resistance welding of the core sheets along the selected lines to establish the junction lines between the core sheets, leaving gaps in the weld lines for passage of forming gas into the cells. This process was faster than the diffusion bonding technique, but still required that the core and face sheets not be loaded into a hot die to avoid premature diffusion bonding of the core sheets to each other. After closing the die, the stack could be purged and pressurized to slightly inflate the stack and separated the sheets from one another so that they would not diffusion bond together where no bonding was desired. The die would then be heated to superplastic temperature and forming gas would be admitted under pressure into the stack to superplastically expand the top and bottom sheets against the walls of the die cavity and stretch the core sheets between the top and bottom sheets to form the desired sandwich structure.
To prevent premature diffusion bonding of the face sheets in the stack with the core sheets, a device is used in the apparatus of the Hayase et al. patents to hold the face sheets spaced apart from the core sheets. Eight separate tooling pieces are shown for this purpose, which increases the cost and complexity of the forming process. For a high rate production operation, it would be preferable to simplify the tooling and enable the parts to be loaded into the die while it is hot, to achieve an increased production rate and lower production cost.
For successful forming to occur, a pressure differential must be established between the face sheet zones and the core sheet zones, and this pressure differential must be equalized over both face sheet zones. Otherwise, the core sheets will form unevenly and will result in excessive thinning.
Heating titanium to a high temperature in the presence of oxygen creates a surface layer of alpha case which is a hard but very brittle composition and is unacceptable in structural parts because of its tendency to crack. Such cracks could grow in a fatigue environment and lead to failure of the part. Consequently, it is desirable to purge oxygen and moisture from the stack of sheets of before heating to elevated temperatures. An ideal process would be one in which the stack of sheets is sealed and purged of oxygen and moisture before loading so the sealed pack could be loaded into a hot die without the danger of alpha case forming before the stack is purged and without using expensive press time to purge the stack and then slowly bring the die up to superplastic temperature.
Another fastening technique, shown in U.S. Pat. No. 4,603,089 to Bampton, uses a CO2 laser to weld sheets in the stack together. Bampton does not teach any way to hold the sheets together while they are being laser welded, and indeed does not disclose any apparatus at all to perform the welding operation. In fact, in a production operation for making a laser-welded multisheet expanded metal sandwich structure, such as that shown in U.S. Pat. No. 5,330,092 to Gregg et al., it is necessary to press the sheets into intimate contact to obtain a quality weld, and to do so with a high speed, efficient, high production rate apparatus in order to benefit from the potential benefits that laser welding has to offer. In addition to exerting a press-up force on the sheets during welding, such an apparatus ideally would protect the weld area from oxidation at high temperature that occurs during laser welding of titanium.
Weld cratering and tight radii at the start and stop of the weld are inherent limitations of laser welding. They are the consequences of the high intensity, narrowly focused nature of the beam, and have in the past resulted in sharp termination points that concentrated stresses at those points which can rip the core sheet when the core is pressurized by forming gas during superplastic forming. The laser naturally produces a xe2x80x9ckeyholexe2x80x9d weldment that forms a crater at the weld termination, severely undercutting the top sheet at the end point of a stitch weld. Such welds weaken the top sheet of the core stack at the weld termination at a point that experiences high stress during inflation by gas pressure during superplastic forming. A production process that optimally utilizes the potential benefits of laser welding would eliminate these weak points at the beginning and terminating ends of the weld.
Accordingly, the present invention provides an improved process for forming multisheet expanded metal sandwich structures, and the structure made thereby, having face sheets diffusion bonded to internal webs wherein the webs have reduced thinning and hence superior strength over similar structures made by prior art processes. The present invention also provides an improved method of cleaning sheets to be diffusion bonded to improve the percentage of well bonded parts produced in production. Another feature of this invention is to provide a traveling laser welding head having a pressure foot for pressing up the sheets in a stack to be laser welded. Another feature of this invention provides an improved process for laser welding a stack of sheets wherein the sheets in the stack are pressed into intimate contact around the region of the weld to ensure good weld quality. In the preferred process of laser welding superplastic sheets for later superplastic forming, stop-off is applied to the sheet interface to prevent diffusion bonding, and the sheets are laser-welded through the stop-off. In yet another aspect, this invention provides a process of forming a multisheet expanded metal sandwich structure by superplastic forming/diffusion bonding wherein the pack of sheets to be formed into the sandwich structure are loaded and unloaded into a superplastic forming die at high temperature. Yet another feature provides an improved process and structure for securely attaching a gas supply line to a pack for superplastic forming/diffusion bonding thereof into a multisheet expanded metal sandwich structure. An improved method laser welds metal sheets together avoiding cratering, undercutting and pointed laser weld end points that can cause concentration of stress in the welded structure and tearing of the sheets when the pack is inflated during superplastic forming. Finally, this invention also provides a method of making a multisheet expanded metal sandwich structure having sealed openings through the sandwich structure for access through the structure for fasteners, fluid or electric lines, control cables or the like.
A preferred process for making an expanded metal sandwich structure includes cleaning at least two metal sheets having superplastic characteristics for forming a core of the sandwich structure to remove metal oxides and residues that would interfere with diffusion bonding of the sheets. At least one surface of at least one of the core sheets is coated with a stop-off compound such as boron nitride to prevent that surface from diffusion bonding to other sheets. The core sheets are placed in a vertical stack, with the stop-off coated surface of the one sheet facing the other sheet. A traveling weld head presses the core sheets together and laser-welds the core sheets through the stop-off along lines which will be along one or more planes located within the metal sandwich structure intermediate the thickness thereof. The laser-welded core sheets form a core pack. A gas pressure line fitting is inserted between the core sheets along at least one edge. The fitting has a through bore that communicates between the exterior of the core sheets and an interior region therebetween. The core pack is seal welded around its peripheral edge and the gas pressure line fitting is welded to the core sheets. The gas pressure line fitting is plugged and the core pack is chemically cleaned to remove metal oxides and residues that would interfere with diffusion bonding of the sheets.
Two additional superplastic metal face sheets are chemically cleaned to remove oily contamination, metal oxides and residues that would interfere with diffusion bonding of the sheets to the core pack. These sheets are placed one each on the top and bottom faces of the core pack. An envelope gas fitting is positioned in a notch in the core pack between the face sheets and the peripheral edges of the face sheets and the core pack are seal welded. The gas fittings are also seal welded between the face sheets to produce a sealed envelope pack enveloping the core pack, producing a full pack, with gas fittings into the core pack and into a face sheet zone between the face sheets and the core pack.
A gas supply tube is connected from a gas supply control system to each of the fittings, and air and moisture are purged from the packs. The packs are pressurized with an inert forming gas such as Argon, the core pack being pressurized to a higher pressure than the full pack. A die is selected having an internal cavity with the same shape as the desired shape of the metal sandwich structure after it is expanded. Preheated to about the superplastic temperature of the metal, the die is opened to receive the full pack. In the die cavity, the temperature of the full pack raises to the superplastic temperature of the metal, and forming gas is injected through the fittings to inflate the envelope pack to the interior walls of the cavity, and to inflate the core pack to the envelope pack. The full pack also inflates around the inserts to produce a sealed hole through the sandwich structure. After forming is completed, the forming gas pressure is reduced to near ambient, and the forming gas pressure in the core pack is reduced to near ambient, just enough to ensure that the cooling of the part does not pull a vacuum that would tend to produce hollows in the part between the webs. The die is opened and the formed pack is removed from the die while still at an elevated temperature above 1600xc2x0 F. The formed pack is allowed to cool below 900xc2x0 F. while remaining connected to the gas supply system, and the gas supply lines are then removed from the gas fittings. Portions of the peripheral flange holding the gas fittings are trimmed off of the formed pack.
If sealed openings through the sandwich structure are needed for fasteners, fluid or electric lines, control cables or the like, a circular laser weld may be made in the full pack before it is superplastically expanded to seal weld around the region where a hole will be cut. The hole can then be cut inside the circular seal weld to produce a sealed opening through the full pack. Insert tubes having a length equal to the height of the die cavity are placed in the sealed holes in the full pack, and the pack forms around the inserts as it inflates, producing sealed openings of the desired opening diameter in the sandwich structure.